Integrated ammonium and nitrate nitrogen supply alters the composition and functionalities of rice rhizosphere bacterial communities and enhances nitrogen use efficiency

Nitrogen (N) is the most important nutrient for plants; however, microbial-mediated N transformation under different nitrogen forms is unclear. This experiment investigates the differential effects of ammonium nitrogen (AN) and nitrate nitrogen (NN) ratios on soil chemical properties, rhizosphere bacterial network, and rice growth. The combined AN and NN (ANNN) treatment enhanced soil pH by 6.9%, soil organic carbon (SOC) by 12%, and microbial biomass nitrogen (MBN) by 60% compared to the control (CK). The Linear discriminant Effect Size (LEfSe) analysis indicated four highly abundant biomarkers of bacterial communities each in CK, NN, and AN treatments, and six highly abundant bacterial communities biomarkers in combined ANNN treatment with maximum effect size and linear discriminant analysis score (LDA) > 4. 16S rRNA genes-predicted functions under PICRUST indicated glutathione metabolism and proteasome, and Tax4fun recorded amino acid metabolism under mixed nutrition of ANNN. Mixed nutrition of ANNN significantly increased the expression levels of genes encoding nitrogen metabolism, including AMT1, NRT2.1, GS1, and GOGAT1, and induced nitrate reductase, nitrite reductase, glutamine synthetase, and glutamate synthase by 39%, 27%, 35%, and 38%, respectively in comparison to CK. In addition, the ANNN treatment promoted rice leaf photosynthetic rate by 37%, transpiration rate by 41%, CO2 exchange rate by 11%, and stomatal conductance by 18% compared to CK, while nitrogen use efficiency (NUE) by 10% and 19% compared to sole AN and NN. These findings suggest that the combination of appropriate ratios of AN and NN can promote bacterial communities' abundance, composition, and functional pathways by improving soil properties to increase NUE and rice growth. This study provides a theoretical basis for the rational application of nitrogen fertilizers and the implication of this approach for future sustainable crop production.